1. CEE 160L – Introduction to Environmental Engineering and Science
Lecture 10
Wastewater

2. Discovery of Biological Wastewater Treatment
Example 5 from Vesilind, p.14
Introduction to Environmental Engineering, Vesilind and Morgan, p. 15

3. Removal Objectives Wastewater Treatment Plant (WWTP)
Sand, grit, debris
Suspended solids
BOD
Pathogens
Nutrients
Nitrogen (ammonia, nitrate)
Phosphorus
Nitrification: NH3 + O2 -> NO3-
Denitrification: NO3- -> N2
This takes place in low-oxygen regions
How do these
remove N?

4. Typical Wastewater Treatment Train
Preliminary treatment
Primary treatment
Secondary treatment
Raw
sewage
Bar
screening
Grit
chamber
Trickling
filter
Activated
sludge
Secondary
clarifier
Return Activated Sludge (RAS)
Primary
clarifier
Solids thickening
Anaerobic digestion
Solids disposal
Disinfection
Discharge
Sometimes there is a trickling filter; sometimes there isn’t. AS is the most common process in secondary treatment!

5. Preliminary Treatment
Purpose
Protect WWTP equipment (e.g., pumps, valves)
Does NOT decrease BOD much
Unit operations (physical treatment)
Bar Screening
Grit Chamber
Comminution
Comminutors are usually downstream of the grit chamber to prevent mechanical abrasion of the cutting bars by the grit.
By settling

6. Riverside, CA WWTP
Bar rack screening pretreatment of the sewage at the inlet of the WWTP.
This removes rags, paper, sticks, etc. that would jam moving parts.
Trash rack
mm opening
Mechanically-cleaned rack
5-40 mm opening

7. Bar Rack Close-up Parallel bars screen out large solids
The chain on the right moves the rake.

8. Grit Chambers
WW flows into chambers where heavy solids (grit) sink to bottom
Detention time ~ minutes
Solids washed to remove organics before deposited in landfill.
Organics
Disagreeable odor
Attract rodents/insects
Remove solids >0.3mm in diameter and specific gravity >2.65

9. Primary (1o) Treatment Purpose
Still offensive!
Primary (1o) Treatment
Purpose
Remove objectionable solids
Remove solids by settling
Remove floating materials, particularly greases, by skimming
Pretreatment often lumped in with 1o treatment
Unit operation
Primary settling tank / primary clarifier (2-3 h)
Physical treatment
1o treatment necessary to increase efficiency
Removes about 45 to 50% of the suspended solids and 25 to 30% of the BOD5 of the incoming WW
Primary
clarifier
Picture is of overflow trough. Note that color of primary effluent is still offensive.

10. Primary Treatment
Circular sedimentation basins remove solids/floatable scum
Quiescent conditions
Flow moves from center to outside edge overflow trough
Note turbulence in center inlet and quiescent flow toward outside edge
Skimmer boom
Skims floating scum, oil, grease

11. Primary Sludge = Sludge from 1o Clarifier
Sludge settling to bottom of 1o clarifiers is collected by rotating rake, which carries sludge to center hopper.
Sludge is then transported to a sludge thickener

12. Secondary (2o) Treatment
Purpose
Primary goal:
Example unit operations
Trickling filter
Activated sludge
Secondary clarifier
Biological treatment
Physical treatment

13. Biological Treatment

14. Example Aerobic Biological Treatment Processes
1. Trickling Filter (Biofilm process)
WW continuously distributed over media
Media: rocks, plastic media
Space between media allow air circulation (i.e., aerobic)
“Filter” is a misnomer; no particle removal.
2. Activated Sludge Process (Suspended growth process)
Aeration tank plus secondary clarifier
Recycles activated sludge (biomass) to maintain a sufficient concentration of biomass in the aeration tank

15. Trickling Filter Sewage sprayed onto bed of support media.
Rocks (heavy) restrict filter depth.
Deeper filters possible with plastic media (light).
Biofilm bacteria grow on media and degrade organics as sewage trickles down.
Water may be recycled, but sludge is not.

16. Activated Sludge Process
Secondary Effluent
Activated Sludge Aerated Basin
Secondary Clarifier
Sludge Thickening/Treatment
Primary Effluent
Q-Qw,
Sout, Xout
Xin, Sin, Q
Return activated sludge
QR, XR
Qw, XR
X = Biomass
S = Substrate = BOD
Q = Volumetric flow rate

17. Activated Sludge Long rectangular aeration basins
Hydraulic residence time
Approximately hr
Biomass residence time
6-30 days
Air injected near bottom of
aeration tanks through diffusers
Controlled by wasting a portion of microorganisms and recycling the rest
Brown color = high biomass concentrations
Conventional AS is plug flow, but CSTRs are also used (decrease tank concentration of toxic compounds).

18. Sludge Age Sludge Age = Mean cell residence time
Also called solids retention time (SRT or θc)
Mass of solids in AS system / solids removal rate
SRT > HRT
Purpose of the return activated sludge (RAS)
To retain microorganisms in system longer (SRT>HRT)
More microorganisms consume more BOD
These assumptions are consistent with those that we made for the MB on biomass.
Get the effluent substrate concentration, S, by combining the definition of SRT with the biomass mass balance.

19. Secondary Clarifier
Notice the blue color after settling of the biological solids
Overflow trough with finger weir located several feet from the edge to allow flow from both sides

20. Secondary Effluent White bubbles
From detergents that passed through treatment without being removed or degraded
Use spray nozzles to suppress foaming

21. Chlorination/Dechlorination
2o effluent is disinfected, often with chlorine.
Pathogen destruction
Because chlorine/chloramine is toxic to aquatic life, water dechlorinated before discharge.
Dechlorinate with sodium bisulfite (NaHSO3), which eliminates chlorine residual.
Dechlorination with sodium bisulfite:
NaHSO3 + Cl2 + H2O  NaHSO4 + 2HCl
NaHSO3 + Cl2 + H2O  NaHSO4 + 2HCl

22. Sludge Treatment Thickening
Procedure to increase the solids content of sludge by removing a portion of water.
Generally accomplished by physical means, including gravity thickening or flotation.
Anaerobic Digestion
Traditional biological method of sludge processing
Anaerobic bacteria convert organics to CO2 and CH4.
Sludge dewatering
Physical process used to reduce the moisture content of sludge so that it is easier to handle
Thickening typically concentrates to about 6% solids.
Flotation involves the release of very fine bubbles, which carry the sludge to the top of the tank, where it is removed.
Sludge dewatering can be accomplished with a belt filter press, which squeezes the sludge between porous cloth belts, where the water can be removed.

23. Anaerobic Digestors
Anaerobic sludge digester (three stories tall) with a hydraulic residence time of about 30 days

24. Drying Beds for Digested Sludge
Digested sludge can be used as agricultural amendment
Improve soil organic content
Used if sludge is “clean”
Must meet standards for heavy metals, toxic non-metals, organic pollutants, and pathogens
Water leaves sludge by drainage (through underlying sand layer) or by evaporation. Can get to solids concentrations of 20 (typical) and 70 percent (atypical).

25. Connection between Wastewater and Drinking Water
Where does treated wastewater go?
Injected into the ground
Discharged to surface water (e.g., lake, river)
Where does drinking water come from?
Groundwater
Surface water
Durham’s treated WW discharges to Ellerbee Creek which eventually goes to Neuse River
A DW source for downstream cities (Raleigh).
Wastewater is highly treated before direct injection to GW.

26. Wastewater Reuse Potable reuse of treated WW is extremely rare
Space shuttle
Colorado River Municipal District
Non-potable reuse
Irrigation (agricultural and municipal), fire protection

28. Septic Field
Drain field for disposal of organic materials readily catabolized by a microbial ecosystem.
The drain field typically consists of an arrangement of trenches containing perforated pipes and porous material (often gravel) covered by a layer of soil to prevent animals and surface runoff from reaching the wastewater distributed within those trenches

29. Some Uganda Water Facts from National Water and Sewer Company
Parts of Kampala such as Kyaliwajala, Kulambiro and most places on hilltops suffer from chronic water shortages
90% and 95% of the water samples taken from protected and treated water supplies, respectively, met national standards for drinking water quality
90% of the collected wastewater of Kampala is discharged without any treatment

31. Schistosomiasis
80–85% of schistosomiasis worldwide is found in sub-Saharan Africa
Praziquantel is the drug of choice for treating schistosomiasis
Praziquantel (Biltricide) is an anthelmintic (expel parasitic worms) effective against flatworms
Vaccine development
Elimination of snails via chemicals or snails that eat or complete with Biomphalaria snails